The present invention relates to a method of making a support of polymeric material having a sealing ring.
EP 0,217,013 B1 discloses a lockable piston-cylinder unit that has at one end a support of polymeric material that serves as a valve housing. The support of polymeric material has an annular seal, which seals off one end of the working space of the cylinder and is held in a groove in the support of polymeric material. The support has a front cover, the axial face of a circumferential cover edge providing a groove side wall by which the annular seal is retained in the receiving groove.
Each of DE 197 14 647 A1, DE 297 20 483 U1 and EP 0,383,303 B1 describes and shows a support of polymeric material for a piston-cylinder unit of like function, wherein the annular seal is injected in place onto the support. In all cases, care has been taken to see that the annular seal cannot slip off the support.
In an unloaded state of operation, the instantaneous pressure on the annular seal in the region of the sealing surface of the support of polymeric material is greater than on the opposing side. This effect is based on the area and pressure ratios of the piston-cylinder unit. In addition, when the piston-cylinder unit is filled, pressure is likewise suddenly applied to the annular seal, which might cause the annular seal to slide off. However, the possibility of slipping or sliding of the annular seal from the support of polymeric material is increased when unfavorable climatic conditions are present, for example, in the transport of piston-cylinder units in a container at ambient temperatures that cause the pretension of the annular seal to be reduced.
A cover-securing device, as in EP 217,013 B1, is rather undesirable with regard to assembly, since an additional part must be provided. In addition, care must be taken to see that the cover also maintains its specified position under all operating conditions.
In the case of seals injection-molded in place within an annular groove, attention has to be paid to compatibility between the material of the seal and the material of the support of polymeric material. Some polymeric materials, because of a highly impervious surface, combine with other types of polymeric materials only with great difficulty.
In the production of supports of polymeric material, two-part molds that part in the region of the groove may be used for the annular seal. Even with the greatest of care, divided molds often leave flash at the parting line, which adversely affects the surface quality of the area to be sealed.
Consideration has been given to whether there are other ways to dispense with axial securing of the sealing ring that faces toward and seals of a working space. It was found that there are sealing materials that are very resistant to changes in temperature but are also of the same composition, and therefore can only be used as a solution in a similarly sensitive application.
The object of the present invention is to provide a support of polymeric material having an annular seal, which is capable of operating even under elevated thermal stress, can be produced at a low cost, and has a high functional reliability.
According to the invention, the foregoing object is attained by-a method of making an injection-molded support of polymeric material having a cylindrical portion having an axis and an end, and an annular groove for a sealing ring located proximate to the end of the cylindrical portion and defined by a base surface, a sealing surface and a ring-retaining surface, the sealing surface being farther from the end of the cylindrical portion than the ring-retaining surface. The base surface and sealing surface are formed by a cavity of a first mold part that separates from the support in the direction of the axis of the cylindrical portion.
The advantages of the method are that very simple molds can be used for injection molding the support, and the base surface and sealing surface are free of mold flashes, since they have no seams due to mold-parting lines.
In some embodiments of the method, a seat portion for a circumferential flange having the ring-retaining surface is formed on the cylindrical portion by the cavity of the first mold, the seat portion including a circumferential rib or groove spaced apart from the base surface. The seat portion is elastically deformed upon separation of the first mold part from the support. The flange is then formed on the seat portion by a cavity in a second mold having two parts that separate radially at a parting plane that includes the axis of the cylindrical portion.
Preferably, the edge of the rib or groove closer to the sealing surface is spaced apart from the sealing surface by a distance (A) greater than the axial width of the base surface. With that relationship maintained, the rib or groove and the flange present shoulders that engage each other so that the flange is mechanically retained on the cylindrical portion. It is desirable for the rib or groove to be formed with a height (H) in the radial direction greater than a maximum possible elastic extension of the flange.
In other embodiments of the method of the present invention, the cavity of the first mold part forms a flange on the cylindrical portion having the ring-retaining surface. The flange is located axially between the end of the cylindrical portion and the base surface, and a part of the cylindrical portion including the flange is elastically deformed when the first mold part is separated from the support. The first mold part forms a plurality of radial slots in the flange to provide free space to receive portions of the flange adjacent the slot when the flange is deformed upon separation the first mold part from the support. The first mold part also forms a circumferential groove in an end portion of the cylindrical portion, the groove being spaced apart radially toward the axis from the base surface and underlying the flange and the base surface. The slots are formed by the first mold part to extend radially between the radially outer end of the flange and the circumferential groove, thus producing a slotted ring portion on the end of the support that is elastically collapsed radially inwardly when the mold part is separated from the support. It is desirable for the transition between the base surface and the flange to be formed to be rounded, which facilitates separating the first mold part from the support.